1. Technical Field
This application relates to managing cache backup and restore.
2. Description of Related Art
Storage devices are employed to store data that is accessed by computer systems. Examples of basic storage devices include volatile and non-volatile memory, floppy drives, hard disk drives, tape drives, optical drives, etc. A storage device may be locally attached to an input/output (I/O) channel of a computer. For example, a hard disk drive may be connected to a computer's disk controller.
As is known in the art, a disk drive contains at least one magnetic disk which rotates relative to a read/write head and which stores data nonvolatilely. Data to be stored on a magnetic disk is generally divided into a plurality of equal length data sectors. A typical data sector, for example, may contain 512 bytes of data. A disk drive is capable of performing a write operation and a read operation. During a write operation, the disk drive receives data from a host computer along with instructions to store the data to a specific location, or set of locations, on the magnetic disk. The disk drive then moves the read/write head to that location, or set of locations, and writes the received data. During a read operation, the disk drive receives instructions from a host computer to access data stored at a specific location, or set of locations, and to transfer that data to the host computer. The disk drive then moves the read/write head to that location, or set of locations, senses the data stored there, and transfers that data to the host.
Advances in semiconductor technology have lead to an increase in the use of a semiconductor solid state drive (also known as a solid state disk or SSD) which uses a flash memory as a storage device, in areas such as computer systems. Thus, in at least some cases there seems to be a trend towards the use of an SSD as a storage device instead of a magnetic disk. In spite of having features such as, for example, a relatively small storage capacity and a relatively high price, the SSD has some other features that can make it more attractive as a storage device than the conventional magnetic disk in at least some cases.
Features that can make SSDs preferable as storage devices are, for example, a fast access rate, high throughput, a high integration density, and stability against an external impact. SSDs can move much larger amounts of data and process far more I/O requests, per time period, than conventional magnetic disks. This allows users to complete data transactions much more quickly.
Furthermore, advances in manufacturing technologies for SSDs may reduce the production costs of SSDs and also increase the storage capacities of SSDs. These developments may provide further incentive to use SSDs in place of magnetic disks in at least some cases.
Solid state disk systems may also comprise communication controllers, such as Fibre Channel (FC) controllers, Ethernet mechanisms, ATA or serial ATA interfaces, or SCSI controllers for managing data communication with external computing devices.
A storage device may also be accessible over a network. Examples of such a storage device include network attached storage (NAS) and storage area network (SAN) devices. A storage device may be a single stand-alone component or be comprised of a system of storage devices such as in the case of Redundant Array of Inexpensive Disks (RAID) groups.
Virtually all computer application programs rely on such storage devices which may be used to store computer code and data manipulated by the computer code. A typical computer system includes one or more host computers that execute such application programs and one or more storage systems that provide storage.
The host computers may access data by sending access requests to the one or more storage systems. Some storage systems require that the access requests identify units of data to be accessed using logical volume (“LUN”) and block addresses that define where the units of data are stored on the storage system. Such storage systems are known as “block I/O” storage systems. In some block I/O storage systems, the logical volumes presented by the storage system to the host correspond directly to physical storage devices (e.g., disk drives) on the storage system, so that the specification of a logical volume and block address specifies where the data is physically stored within the storage system. In other block I/O storage systems (referred to as intelligent storage systems), internal mapping technology may be employed so that the logical volumes presented by the storage system do not necessarily map in a one-to-one manner to physical storage devices within the storage system. Nevertheless, the specification of a logical volume and a block address used with an intelligent storage system specifies where associated content is logically stored within the storage system, and from the perspective of devices outside of the storage system (e.g., a host) is perceived as specifying where the data is physically stored.
In contrast to block I/O storage systems, some storage systems receive and process access requests that identify a data unit or other content unit (also referenced to as an object) using an object identifier, rather than an address that specifies where the data unit is physically or logically stored in the storage system. Such storage systems are referred to as object addressable storage (OAS) systems. In object addressable storage, a content unit may be identified (e.g., by host computers requesting access to the content unit) using its object identifier and the object identifier may be independent of both the physical and logical location(s) at which the content unit is stored (although it is not required to be because in some embodiments the storage system may use the object identifier to inform where a content unit is stored in a storage system). From the perspective of the host computer (or user) accessing a content unit on an OAS system, the object identifier does not control where the content unit is logically (or physically) stored. Thus, in an OAS system, if the physical or logical location at which the unit of content is stored changes, the identifier by which host computer(s) access the unit of content may remain the same. In contrast, in a block I/O storage system, if the location at which the unit of content is stored changes in a manner that impacts the logical volume and block address used to access it, any host computer accessing the unit of content must be made aware of the location change and then use the new location of the unit of content for future accesses.
One example of an OAS system is a content addressable storage (CAS) system. In a CAS system, the object identifiers that identify content units are content addresses. A content address is an identifier that is computed, at least in part, from at least a portion of the content (which can be data and/or metadata) of its corresponding unit of content. For example, a content address for a unit of content may be computed by hashing the unit of content and using the resulting hash value as the content address. Storage systems that identify content by a content address are referred to as content addressable storage (CAS) systems.
Some storage systems receive and process access requests that identify data organized by file system. A file system is a logical construct that translates physical blocks of storage on a storage device into logical files and directories. In this way, the file system aids in organizing content stored on a disk. For example, an application program having ten logically related blocks of content to store on disk may store the content in a single file in the file system. Thus, the application program may simply track the name and/or location of the file, rather than tracking the block addresses of each of the ten blocks on disk that store the content.
File systems maintain metadata for each file that, inter alia, indicates the physical disk locations of the content logically stored in the file. For example, in UNIX file systems an inode is associated with each file and stores metadata about the file. The metadata includes information such as access permissions, time of last access of the file, time of last modification of the file, and which blocks on the physical storage devices store its content. The file system may also maintain a map, referred to as a free map in UNIX file systems, of all the blocks on the physical storage system at which the file system may store content. The file system tracks which blocks in the map are currently in use to store file content and which are available to store file content.
When an application program requests that the file system store content in a file, the file system may use the map to select available blocks and send a request to the physical storage devices to store the file content at the selected blocks. The file system may then store metadata (e.g., in an inode) that associates the filename for the file with the physical location of the content on the storage device(s). When the file system receives a subsequent request to access the file, the file system may access the metadata, use it to determine the blocks on the physical storage device at which the file's content is physically stored, request the content from the physical storage device(s), and return the content in response to the request.
In general, since file systems provide computer application programs with access to data stored on storage devices in a logical, coherent way, file systems hide the details of how data is stored on storage devices from application programs. For instance, storage devices are generally block addressable, in that data is addressed with the smallest granularity of one block; multiple, contiguous blocks form an extent. The size of the particular block, typically 512 bytes in length, depends upon the actual devices involved. Application programs generally request data from file systems byte by byte. Consequently, file systems are responsible for seamlessly mapping between application program address-space and storage device address-space.
File systems store volumes of data on storage devices, i.e., collections of data blocks, each for one complete file system instance. These storage devices may be partitions of single physical devices or logical collections of several physical devices. Computers may have access to multiple file system volumes stored on one or more storage devices.
File systems maintain several different types of files, including regular files and directory files. Application programs store and retrieve data from regular files as contiguous, randomly accessible segments of bytes. With a byte-addressable address-space, applications may read and write data at any byte offset within a file. Applications can grow files by writing data to the end of a file; the size of the file increases by the amount of data written. Conversely, applications can truncate files by reducing the file size to any particular length. Applications are solely responsible for organizing data stored within regular files, since file systems are not aware of the content of each regular file.
Files are presented to application programs through directory files that form a tree-like hierarchy of files and subdirectories containing more files. Filenames are unique to directories but not to file system volumes. Application programs identify files by pathnames comprised of the filename and the names of all encompassing directories. The complete directory structure is called the file system namespace. For each file, file systems maintain attributes such as ownership information, access privileges, access times, and modification times.
File systems often utilize the services of operating system memory caches known as buffer caches and page caches. These caches generally consist of system memory buffers stored in volatile, solid-state memory of the computer. In this context, caching is a technique to speed up data requests from application programs by saving frequently accessed data in memory for quick recall by the file system without having to physically retrieve the data from the storage devices. Caching is also useful during file writes; the file system may write data to the memory cache and return control to the application before the data is actually written to non-volatile storage. Eventually, the cached data is written to the storage devices.
The state of the cache depends upon the consistency between the cache and the storage devices. A cache is “clean” when its contents are exactly the same as the data stored on the underlying storage devices. A cache is “dirty” when its data is newer than the data stored on storage devices; a cache becomes dirty when the file system has written to the cache, but the data has not yet been written to the storage devices. A cache is “stale” when its contents are older than data stored on the storage devices; a cache becomes stale when it has not been updated to reflect changes to the data stored on the storage devices.
In order to maintain consistency between the caches and the storage devices, file systems perform “flush” and “invalidate” operations on cached data. A flush operation writes dirty cached data to the storage devices before returning control to the caller. An invalidation operation removes stale data from the cache without invoking calls to the storage devices. File systems may flush or invalidate caches for specific byte-ranges of the cached files.
Many file systems utilize data structures mentioned above called inodes to store information specific to each file. Copies of these data structures are maintained in memory and within the storage devices. Modes contain attribute information such as file type, ownership information, access permissions, access times, modification times, and file size. Modes also contain lists of pointers that address data blocks. These pointers may address single data blocks or address an extent of several consecutive blocks. The addressed data blocks contain either actual data stored by the application programs or lists of pointers to other data blocks. With the information specified by these pointers, the contents of a file can be read or written by application programs. When application programs write to files, data blocks may be allocated by the file system. Such allocation modifies the inodes.
Additionally, file systems maintain information, called “allocation tables”, that indicate which data blocks are assigned to files and which are available for allocation to files. File systems modify these allocation tables during file allocation and de-allocation. Most modem file systems store allocation tables within the file system volume as bitmap fields. File systems set bits to signify blocks that are presently allocated to files and clear bits to signify blocks available for future allocation.